This invention is related to apparatus for melting and dispensing thermoplastic material, and, more particularly, to an apparatus for melting reactable moisture curable thermoplastic material within a moisture-proof, sealed container and discharging the molten thermoplastic material from the container to one or more adhesive dispensers.
Thermoplastic adhesives, or so-called hot melt adhesives, are commonly used in bonding a wide variety of materials. Hot melt adhesives have been employed in the assembly of packages, automobile parts, electrical equipment and a variety of other applications which require a supply of relatively large quantities of adhesive.
In order to meet the demands of these types of applications, apparatus have been commercialized such as disclosed, for example, in U.S. Pat. Nos. 3,792,801; 3,964,645; and 4,200,207, each of which is assigned to the assignee of this invention. Apparatus of this general type consist of a hopper having an open top for receiving solid hot melt adhesive, and a heated grid positioned at the bottom of the hopper which operates to melt the solid adhesive and allow it to flow in liquid form into a container or reservoir. A heated pump is located internally of the liquid reservoir through which the melted hot melt adhesive is pumped via a heated conduit and heated dispenser to the surface to be bonded
When using apparatus of the type described above, individual bars of hot melt adhesive are dumped into the open hopper periodically during a production run. Accordingly, the hot melt adhesive in the hopper is exposed to the atmosphere for a period no less than the time required to melt the adhesive, and may be exposed for many hours if the equipment is used intermittently. Although exposure to the atmosphere does not affect some types of hot melt adhesives, the performance and bonding characteristics of reactable adhesives such as moisture curable hot melt adhesives are adversely affected by contact with atmosphere prior to use. It is necessary to maintain such adhesives in an inert atmosphere such as within a moisture-proof container, and provide for dispensing from the container immediately prior to use, to ensure optimum performance of such adhesives. The prior art apparatus described above are thus unsuitable for dispensing rapidly degradable reactive adhesives.
Hot melt adhesive melting and dispensing systems particularly adapted for handling reactable, moisture curable hot melt adhesives have been proposed, and one system is disclosed in U.S. Pat. No. 4,586,636. In systems of this general type, the moisture curable hot melt adhesive is stored and shipped in bulk within moisture-proof, thermally conductive containers. The containers are formed with an externally threaded neck at one end having a discharge outlet, and a closure seal at the opposite end. The closure seal contacts the hot melt adhesive and is axially movable within the interior of the container toward the neck.
The apparatus disclosed in Pat. No. 4,586,636 for melting and discharging the moisture curable adhesive from the container includes a thermally conductive platen formed with a throughbore having internal threads adapted to mate with the neck of the container. The platen is mounted upon a heating block having heating elements for raising the temperature of the platen. The container is mounted on the top surface of the platen by threading its neck into the throughbore of the platen, and affixing a clamp to the base of the container.
The platen is heated by the heating block to a temperature which melts the adhesive at the end of the container resting on the platen. A piston-actuated ram then contacts the closure seal at the opposite end of the container, and moves it axially within the interior of the container to force the molten adhesive through the discharge outlet of the neck. The molten adhesive flows from the container outlet into the throughbore of the platen, and then through an outlet passageway formed in the heating block. The outlet passageway is connected to one or more dispensers which apply the adhesive to a substrate.
The ram continues its axial movement toward the neck of the container until all of the adhesive is discharged and the container is empty. Any drippage of adhesive which occurs in the course of replacing an empty container with a full one is caught by a tray interposed between the heating block and platen.
There are a number of problems with apparatus of the type disclosed in Pat. No. 4,586,636 which limit their effectiveness and ease of use. One problem relates to a failure to thermally isolate the adhesive from variations in the temperature of the platen and heating block. As disclosed in the 4,586,636 patent, for example, the heating block is directly mounted to the thermally conductive platen with no insulation or other means of thermally isolating the adhesive flowing through the outlet passageway of the heating block to the adhesive dispensers. Failure to thermally insulate the adhesive flowing into the outlet passageway of the heating block subjects the adhesive to temperature fluctuations of the heating block and platen which can occur during the course of a production run. As a result, the hot melt adhesive passing through the outlet of the heating block to the dispensers can become charred or too viscous for application to the substrate.
Another problem with apparatus such as disclosed in U.S. Pat. No. 4,586,636 involves the inefficient transfer of heat to the stream of adhesive as it flows through the discharge opening in the neck of the container into the throughbore in the platen.
The throughbore in the platen which receives the neck of the container forms a single, large passageway having a diameter approximately equal to the diameter of the neck of the container. When the apparatus is shut down, or operated intermittently, the adhesive within the throughbore of the platen cools as the platen temperature is decreased. A slug of viscous or semi-solid adhesive can thus be formed in the throughbore of the platen which must be heated upon start up of the apparatus. Unfortunately, the large diameter annular wall formed by the passageway in the platen contacts only the outermost portion of the slug of hot melt adhesive. The interior portion of the slug does not directly contact a heated surface and can pass through the heating block to the dispensers without being uniformly heated to the proper temperature.
Another problem with apparatus of the type described above relates to spillage or drippage of hot melt adhesive in the course of removing an empty container from the platen. The 4,586,636 patent, for example, discloses a spillage tray intended to catch such drippings which is positioned between the platen and heating block. This tray protects only the heating block from spillage and allows adhesive to drip onto the top of the platen located immediately beneath the container neck as it is removed from the platen. A buildup of excess adhesive on the platen reduces the heat transfer efficiency between the platen and container, and presents difficult cleaning problems.